1. Field of Invention
This invention relates to the field of liquid crystal displays. More particularly this invention is directed to multi-color reflective liquid crystal displays that reflect multiple colors from individual cells.
2. Description of Related Art
Current reflective liquid crystal displays (LCDs) can provide reduced power consumption, lighter weight, thinner packages, and better adaptability to a wider range of ambient conditions (e.g., in-doors and out-doors) than transmissive or emissive flat panel displays (FPDs) can. These features make reflective LCDs suitable for many applications where portability and/or viewability under high ambient illumination are desired. Personal document readers (PDR), personal information tools (PIT), and hand-held maps and manuals are exemplary applications for these reflective LCDs.
The reflective nature of reflective LCDs can reduce power consumption by one-half by eliminating backlight requirements. Further reduction in power consumption in reflective LCDs can be achieved in polymer-stabilized cholesteric texture (PSCT) liquid crystal displays. These polymer-stabilized cholesteric texture liquid crystal displays comprise a cholesteric liquid crystal medium which contains a stabilizing polymer network.
FIGS. 1(a)-1(c) illustrate the operation of such a polymer-stabilized cholesteric texture liquid crystal display 10, as described in "Reflective Color Displays for Imaging Applications," by G. P. Crawford et al., Proceedings of the IS & T/SID 1995 Color Imaging Conference: Color Science, Systems and Applications, pp. 52-58, incorporated herein by reference in its entirety. The display 10 comprises a pair of transparent substrates 12, a pair of transparent electrodes 14, and a polymer stabilized cholesteric liquid crystal medium 16 located between the electrodes 14. A polymer network 18 stabilizes the liquid crystal material 20.
As shown in FIG. 1(a), in the voltage off-state, the planar texture is stable and the helical axes of the cholesteric liquid crystal material 20 are substantially perpendicular to the surface 22 of the substrate 12 on which natural or artificial light impinges. The planar texture selectively reflects incident light centered at the Bragg wavelength, .lambda..sub.B =nP, where n is the average index of refraction of the liquid crystal material 20 and P is the pitch length of the helical structure of the liquid crystal material 20. The pitch P can be selectively varied by adding chiral agents to the liquid crystal material 20. The chiral agents affect the pitch of the cholesteric liquid crystal material 20, and thus also the Bragg wavelength .lambda..sub.B.
As shown in FIG. 1(b), when a low voltage is applied between the electrodes 14 by a voltage source 24, the planar texture of the cholesteric liquid crystal material 20 is transformed into a focal conic texture, in which the helical axes of the liquid crystal are randomly aligned. In this state, the cell is substantially light transparent. If the applied voltage is then removed, the focal conic texture remains fixed due to the stabilizing effect of the polymer network 18. As shown in FIG. 1(c), if a greater field is applied to the display, the cholesteric liquid crystal material 20 becomes completely aligned and completely transparent.
The response of the polymer cholesteric stabilized liquid crystal material 16 to the release of the applied field is dependent on the rate of release of this field. Quickly releasing this field causes the cholesteric liquid crystal material 20 to relax back to the planar texture shown in FIG. 1(a). If the field is released more slowly, then the cholesteric liquid crystal material 20 will relax back to the focal conic texture shown in FIG. 1(b). This bistable memory capability of polymer-stabilized cholesteric texture LCDs can significantly reduce the power consumption of the displays in many applications, because the displays consume no power when viewed and only need to be powered for short periods of time to change the displayed image.
FIGS. 2(a)-2(d) show another type of known reflective color display device, a holographically structured polymer dispersed liquid crystal (HPDLC) display 30. These displays are also described in the incorporated Crawford reference. Holographically structured polymer dispersed liquid crystal displays are formed using optical interference techniques applied to a mixture of liquid crystal material and photocurable polymer material. This technique forms fringe planes 33 of liquid-crystal-filled droplets at predetermined positions within a polymer matrix separated by a plurality of polymer-rich planes, 35. As a result, the liquid crystal droplet densities are spatially modulated in the direction perpendicular to the planar structure. As shown in FIG. 2(a), a holographically structured polymer dispersed liquid crystal material 32 is sandwiched between a pair of transparent electrodes 34 and a pair of transparent substrates 36. In particular, in each liquid-crystal-rich plane 33, a plurality of liquid crystal droplets 38 are dispersed in a network of the polymer 39. In contrast, in the polymer-rich planes 35, there is essentially only the network of the polymer 39. In the field-off state shown in FIGS. 2(a) and 2(b), the liquid crystal molecules in the liquid crystal droplets 38 dispersed in the polymer 39 are randomly oriented, or misaligned, within the liquid crystal droplets 38. The effective refractive index of the droplets 38 is therefore significantly higher than the index of refraction, n.sub.p, of the polymer matrix 39. The directions of the extraordinary index of refraction, n.sub.e, and the ordinary index of refraction, n.sub.o, of the liquid crystal droplets 38 are shown. Consequently, the holographically structured polymer dispersed liquid crystal display 30 reflects light at the Bragg wavelength .lambda..sub.B. The reflectance under ambient illumination conditions for these holographically structured polymer dispersed liquid crystal displays 30 is larger than that for the polymer-stabilized cholesteric texture display 10 shown in FIGS. 1(a)-1(c).
When voltage is applied across the liquid crystal display 30, as shown in FIGS. 2(c) and 2(d), the liquid crystal molecules align within the liquid crystal droplets 38, as shown in FIG. 2(d). As a result, the index of refraction of the liquid crystal droplets 38 approximately equals the index of refraction of the polymer matrix 39 along the direction of light propagation. As shown, n.sub.e is parallel to the incident and reflected light direction, and n.sub.o is perpendicular to the incident and reflected light direction. The periodic refractive index modulation vanishes if n.sub.o of the liquid crystal material in the liquid crystal droplets 38 approximately equals the index of reflaction n.sub.p of the polymer matrix 39. In this state, the holographically structured polymer dispersed liquid crystal display 30 is essentially light transparent and does not reflect or diffract the light incident on the holographically structured polymer dispersed liquid crystal display 30.
The effective refractive index of the liquid crystal droplets 38 can be varied with applied voltage, enabling the reflected light intensity to be controlled electrically.
In addition, the spectral reflectance of the holographically structured polymer dispersed liquid crystal display 30 can be selectively controlled by fabrication. To date, such holographically structured polymer dispersed liquid crystal displays 30 represent an especially promising reflective technology because of their high peak reflectance capability.
Holographically structured polymer dispersed liquid crystal reflective displays are described in detail in U.S. Pat. No. 6,133,971, incorporated herein by reference in its entirety.